Why Backdrivability Matters in Humanoid Actuator Design

In this article, we will take a closer look at backdrivability, one of the key performance indicators in humanoid actuators.

For many years, industrial robots have evolved around two central priorities: accuracy and repeatability. Their primary role has been to perform predefined processes reliably and consistently.

Tasks such as welding automobile bodies, transferring semiconductor wafers, or stacking pallets in precise positions require stable and predictable motion. In these environments, the design direction has been quite clear. The goal is to execute repetitive operations with minimal error while maintaining consistent and reliable performance.

The environments where industrial robots operate are typically highly structured. The range of motion is defined during the design stage, and each movement and position is programmed in advance. Because the workspace is controlled and predictable, situations where a person suddenly enters the robot’s working area or unexpected external forces are applied are generally prevented by design. Humans and robots usually do not share the same workspace directly. Instead, safety fences or physical partitions are commonly used to separate the spaces entirely.

This does not mean safety is ignored. Rather, the philosophy is to prevent collisions in the first place by separating the environments. Since collision scenarios are not considered normal operating conditions, actuator compliance or backdrivability is not typically emphasized as a primary design metric. For example, even a deviation of a few millimeters in a welding point can lead to quality issues, and small vibrations during wafer transfer can directly affect manufacturing yield.

1. Humanoid Robots Designed to Resemble Humans

So what about humanoid robots?

Humanoid robots, designed to mimic the structure of the human body, have characteristics that are fundamentally different from traditional industrial robots built for repetitive tasks.

Conventional industrial robots typically operate within a fixed radius inside a specific production cell. Their installation position rarely changes. If they need to be deployed in a different process, the equipment must be relocated and reinstalled, which leads to additional cost and downtime.

Humanoid robots, on the other hand, are structurally designed with mobility in mind. They are intended to walk on two legs, climb stairs, and move autonomously through a workspace. One of the key advantages of this design is that the hardware itself does not need to be rearranged when the environment changes. Of course, this introduces new challenges related to learning and control, but it also highlights how fundamentally different the starting point of humanoid robot design is compared with conventional industrial robots.

While industrial robots are optimized to deliver maximum performance within a specific and controlled environment, humanoid robots are designed to adapt and operate across a variety of environments. Designing for mobility means the robot must continuously interact with the physical world around it.

Floor inclines, unexpected obstacles, potential contact with people, and sudden external impacts all become part of the design assumptions. Because of these conditions, the engineering requirements for humanoid robots inevitably differ from those of traditional industrial robots.

2. Impressive Motion Capabilities, But Where Does the Impact Go?

In recent years, humanoid robots have advanced rapidly, reaching the point where they can perform backflips and other complex gymnastic movements. These motions generate significant instantaneous impact loads on the robot’s joints. When a human lands from a height, the knee absorbs and distributes the impact through the combined action of cartilage, muscles, and ligaments.

A robotic joint, however, is a mechanical structure composed of gears, bearings, housings, and motors. The impact generated during landing ultimately travels through these components. The key question then becomes where that energy is absorbed and how it is distributed within the system.

If impact loads repeatedly concentrate on internal gear tooth surfaces and bearing contact points, the long term durability of the joint can be affected. For this reason, the way a joint responds to external torque is not simply a matter of performance. It is also closely related to the structural lifespan of the system. One of the factors that directly influences this response behavior is the actuator’s backdrivability.

3. Comparing Joint Behavior Based on Backdrivability

So how does backdrivability actually appear during real motion?

Let us take a look at this through a video demonstration.

As you can see in the video, even when the actuator is powered off and in a stationary state, the joint can be moved smoothly when a person pushes or pulls it directly. In many high reduction industrial robot systems, this kind of motion is typically difficult to achieve.

In general, backdrivability tends to decrease as the gear reduction ratio increases. As the reduction ratio becomes larger, a greater amount of torque is required to transmit force from the output shaft back to the input shaft.

A high reduction ratio offers a clear advantage. It allows a relatively small motor torque to generate a large output torque. At the same time, the output shaft rotates more slowly, which helps stabilize control and makes it easier to maintain accurate positioning.

However, the higher the reduction ratio becomes, the more difficult it is for externally applied forces at the output shaft to propagate back toward the input side. In other words, the structure becomes rigid and resistant to external forces, but its ability to yield or respond naturally to those forces becomes weaker.

In contrast, when a system has good backdrivability, an external force applied at the output shaft can move the joint, and that force travels through the reduction mechanism toward the input shaft, causing the motor to rotate as well. This is the situation in which backdriving occurs.

This behavior is possible because the actuator is designed so that external torque generated at the output can be transmitted relatively smoothly through the internal gear structure of the reducer. In other words, the system is built so that external forces are not significantly blocked by internal friction or gear resistance and can mechanically propagate back through the drivetrain.

At this point, it is also useful to consider the relationship with backlash. Backlash refers to the clearance between meshing gear teeth. Reducing backlash generally improves positioning accuracy and responsiveness. Backdrivability, on the other hand, describes the ability of an external force to drive the output shaft backward and rotate the input shaft.

Structures with high reduction ratios and larger internal friction are often advantageous for reducing backlash and increasing rigidity. However, they also tend to exhibit lower backdrivability.

As a result, high reduction ratio mechanisms are well suited for applications that require slow, strong, and stable motion. But for humanoid joints that must respond flexibly to external impacts or physical interaction, a different design consideration becomes necessary. This difference represents one of the key distinctions between humanoid actuator design and traditional industrial robot systems.

<Bonsystems Humanoid Actuator, BCSA V4 Series Image>

4. Joint Characteristics Required by Humanlike Structures

The true value of humanoid robots does not lie simply in their humanlike appearance or their ability to walk on two legs. Their real advantage is the ability to operate in environments originally designed for humans, using those spaces without requiring fundamental changes.

Actions such as turning a door handle, climbing stairs, lifting a box, or using tools all take place in environments built around the structure of the human body. For this reason, humanoid actuator design ultimately comes down to how effectively the mechanical system can reproduce the characteristics of human joints.

Human joints are not simply structures that transmit force. Nor are they systems that only resist external forces rigidly. When pushed, they yield. When impacted, they absorb and distribute the shock. When necessary, they can also provide strong support. This behavior comes from the complex compliance created by muscles, tendons, and ligaments working together.

The anthropologist John Napier once explained that the evolution of the human hand, capable of both precise manipulation and powerful gripping, played an important role in the development of the human brain. The Hand (Revised by R. H. Tuttle), Princeton University Press.

What matters here is not only precision or rigidity, but the ability to modulate force and respond to external forces depending on the situation. This is precisely why backdrivability becomes such an important design consideration in humanoid actuators.

5. Backdrivability and Torque: Where Engineering Meets Experience

When designing humanoid actuators, the discussion often converges on one central question.

“How can we maintain the required torque while still achieving sufficient backdrivability?”

Reducing the gear ratio can improve backdrivability. However, doing so also makes it more difficult to secure the output torque needed for many robotic applications. On the other hand, increasing the reduction ratio allows a relatively small motor to generate large output torque. The tradeoff is that external forces applied at the output side have a harder time propagating back through the system. The structure becomes more rigid, but its compliance decreases.

Addressing this challenge requires careful consideration of many engineering factors. The reduction ratio itself is only one part of the equation. Gear geometry, the structural integration of the motor and reducer, internal friction characteristics, and component manufacturing accuracy all influence the final behavior of the actuator.

For this reason, humanoid actuators cannot be evaluated solely based on torque density, price competitiveness, or durability. They must be designed with the understanding that the system will continuously interact with the physical world as part of a dynamic mechanical environment.

In humanoid robots, walking, landing, object manipulation, and contact with humans can occur within the same operational context. In such systems, the question is not only how strong the joint can be, but also how it responds to external forces.

Balancing these two characteristics is one of the central challenges in actuator design. Achieving this balance through thoughtful engineering is likely to become one of the key technologies shaping the future of humanoid robotics.

The BCSA SERIES is a humanoid actuator developed based on this design philosophy. Rather than simply increasing the gear reduction ratio to obtain higher torque, the actuator was designed from the early stages of reduction ratio configuration and internal gear structure development, taking into account how humanoid joints respond to external forces.

“Are you currently evaluating joint designs that require backdrivability for your humanoid robot project?”

Product inquiries and technical consultations can be submitted through the contact form on our website. If you share information such as the target joint location, required torque range, desired reduction ratio, operating environment, purchase quantity, and estimated annual production volume, we can review your requirements from a design perspective and recommend a suitable humanoid actuator model.

6. Frequently Asked Questions